1. Field of the invention
The present invention relates in general to a Raman laser oscillation, and more particularly to method and apparatus for Raman laser oscillation using both stimulated Brillouin scattering and stimulated Raman scattering.
2. Description of the Prior Art
As well noted to those skilled in the art, in a Raman laser oscillator using stimulated Raman scattering, a pumping beam generated by a pumping resonator is scattered in a gas cell, thus to change the wavelength of a laser beam and to output a laser beam having a desired wavelength. Such laser oscillators using stimulated Raman scattering have been thus wide used as various industrial measuring devices.
With reference to FIG. 1, there is shown a prior art Raman laser oscillator using stimulated Raman scattering such as disclosed in U.S. Pat. No. 4,821,272. This Raman laser oscillator includes a methane gas cell 2 which is placed on the front of a laser output mirror 7 of a laser resonator such that the laser oscillator uses a wavelength changing effect of the laser beam caused by stimulated Raman scattering (SRS) generated in the gas cell 2.
In the above Raman laser oscillator, a resonance reflection mirror 6 serves to reflect a pumping beam of a pumping source (not shown). The laser output mirror 7 of the laser resonator serves to reflect, in cooperation with the reflection mirror 6, the pumping beam in the resonator, thus to amplify the pumping beam. This laser output mirror 7 is also coated with a dielectric on a surface thereof toward the surface 4A of a laser rod 4 such that it has a high reflectance for the pumping beam and reflects the pumping beam, reflected by the gas cell 2, to the gas cell 2. An optical Q-switch 5 is placed between the two mi trots 6 arid 7 and is an optical element which serves to shorten the laser pulse width but to strengthen the laser intensity and preferably uses a saturable absorber. When using the saturable absorber, the Q-switch 5 provides a pulse width of 10-20 ns. The laser rod 4 is placed between the laser output mirror 7 and the Q-switch 5, and serves to amplify the laser beam reciprocating between the two mi trots 6 and 7. When using Nd:YAG crystal as the laser rod 4, the generated laser beam has a wavelength of 1.06 .mu.m. The laser oscillator further includes a condensing lens 3 which is placed between the gas cell 2 and the laser output mirror 7 in order to condense the laser beam generated by the laser resonator to the gas cell 2. When the pumping beam is condensed to the gas cell 2, the stimulated Raman scattering is induced by interrelation between the condensed pumping beam and molecular vibration of the gas medium molecules inside the gas cell 2, thus to change the wavelength of the pumping beam.
That is, the pumping beam is scattered by the stimulated Raman scattering is increased in its wavelength, otherwise stated, its frequency is decreased. For example, when methane gas (CH.sub.4) is used as the gas medium of the gas cell 2, the wavelength 1.06 .mu.m of the pumping beam is increased to 1.54 .mu.m. The gaseous material or a fluid material, such as methane, in the gas cell 2 is generally named as a Raman medium. The Raman medium is generally selected from a group of methane, deuterium (D.sub.2) and hydrogen (H.sub.2). The laser beam, changed in its wavelength by the stimulated Raman scattering, is outputted from a collimating lens I in the form of a parallel beam.
However, it has been noted that such a prior art Raman laser oscillator using stimulated Raman scattering has the following problems.
First, the prior art Raman laser oscillator using stimulated Raman scattering has a relatively higher larger loss due to stimulated Brillouin scattering induced together with the stimulated Raman scattering in the gas cell 2. This stimulated Brillouin scattering is disclosed in detail in page 1072 of W. Kaiser & M. Maler, "Stimulated Rayleigh, Brillouin and Raman Spectroscopy" Laser Handbook, F. T. Arrecchi and E. O. Shulz--Dubols, eds., Laser Handbook, (North Holland Publishing Co., Amsterdam, 1972). This stimulated Brillouin scattering is always induced together with the stimulated Raman scattering in the gas cell 2. The stimulated Brillouin scattering is a non-linear optical phenomenon in that a laser beam is scattered by an acoustic wave generated in the Raman medium in the gas cell. The laser beam affected and scattered by the stimulated Brillouin scattering scarcely shows change of wavelength and necessarily reversely travels in an incident direction of the laser beam to the gas cell 2, that is, in the backward direction of the gas cell 2, since the laser beam scattered by the stimulated Brillouin scattering or the stimulated Brillouin scattered laser beam shows a phase conjugate characteristic. The phase conjugate characteristic of the laser beam scattered by the stimulated Brillouin scattering is disclosed in detail in B. Ya. Zel'aldrich, N. F. Piliptsky, V. V. Shkunov, "Principles of Phase Conjugation", Springer-Verag (1985). The laser beam scattered by the stimulated Brillouin scattering returns to laser output mirror 7 through the condensing lens 3 by which the laser beam was condensed to the gas cell 2. However, the laser beam scattered by the stimulated Brillouin scattering and returning to the laser output mirror 7 is mostly reflected by the laser output mirror 7 since it has a wavelength which is nearly equal to that of the pumping beam, such that it is not introduced into the laser resonator. The stimulated Brillouin scattering of the prior art Raman laser oscillator thus causes a laser output deterioration or a laser output loss. Otherwise stated, in the prior art Raman laser oscillator using the stimulated Raman scattering, the laser output mirror 7 interferes with the stimulated Brillouin scattering as well as the phase conjugate of the laser beam scattered by the stimulated Brillouin scattering, thus to serve to interfere with the laser output of the Raman laser oscillator and to cause deterioration of efficiency of the oscillator and weakening of the output laser intensity.
Second, it is not easy to achieve a desired optical alignment of optical elements of the prior art Raman laser oscillator using stimulated Raman scattering. This is caused by the fact that the prior art Raman laser oscillator should have the laser output mirror 7, and the more the number of the elements of the laser oscillator is, the more difficult the optical alignment of the elements of the laser oscillator is. When a desired precise optical alignment of the elements of the oscillator is not achieved, the laser output of the oscillator is inevitably deteriorated. On the other hand, even when the desired optical alignment of the elements of the oscillator is achieved, the alignment is apt to be distorted by bad effect given to the elements by their circumstances, such as mechanical vibration and mechanical shock, in accordance with lapse of time. In addition, such a distortion of the optical alignment of the elements intends to be in proportion to the number of the elements. Thus, it is preferred to reduce the number of the elements of the Raman laser oscillator in order to achieve and maintain the desired optical alignment of the elements of the oscillator,
In this regard, the laser output and efficiency of the Raman laser oscillator using stimulated Raman scattering will be increased by additionally preferably using the stimulated Brillouin scattering induced together with the stimulated Raman scattering of the laser beam in the gas cell. In addition, reduction of the number of the elements of the Raman laser oscillator will easily achieve and maintain the desired optical alignment of the elements of the oscillator.